1. Volume 100, Issue 1, Pages 157-168 (January 2000)
Stem Cells 
Irving L Weissman 
Cell 
Volume 100, Issue 1, Pages (January 2000)
DOI: /S (00)81692-X

2. Figure 1 Hematopoietic Stem and Progenitor Cell Lineages
(A) Model of HSC self-renewal and differentiation. HSCs can either be LT-HSCs (Thy-1.1loLin−Sca1+c-Kit+), highly self-renewing cells that reconstitute an animal for its entire lifespan, or ST-HSCs (Thy-1loLin−Mac1loSca1+c-Kit+), which reconstitute the animal for a limited period. ST-HSCs differentiate into multipotent progenitors, which have the ability to differentiate into oligolineage progenitors and ultimately give rise to differentiated progeny. ST-HSCs cannot give rise to LT-HSCs, but can self-renew for a limited time or give rise to MPPs (Thy-1.1loLin−Sca1+ c-Kit+Mac1loCD4lo). Each stage of differentiation involves functionally irreversible maturation steps.
(B) Model of HSC differentiation. Included in the progeny of mouse HSCs are two kinds of oligolineage-restricted cells: common lymphocyte progenitors (CLPs), which can be isolated with the phenotype Lin−IL-7R+Thy1.1−Sca-1loc-Kitlo, and which at the clonal level are restricted to give rise to T lymphocytes, B lymphocytes, and natural killer (NK) cells; and CMPs, which are progenitors for the myeloerythroid lineages. CMPs give rise to cells that include myelomonocytic progenitors (GMPs) and megakaryotic/erythroid progenitors (MEPs). All of these populations are separable as pure populations using cell surface markers.
Cell  , DOI: ( /S (00)81692-X)

3. Figure 2 Model of the Alternative Fates of HSCs
The progeny of HSC cell division can self-renew, differentiate (thereby becoming a more committed progenitor), undergo apoptosis (PCD), or undergo mobilization. Under certain conditions, HSCs migrate and seed other organs. The number of HSCs is in part regulated by PCD. Adapted from Domen and Weissman 1999.
Cell  , DOI: ( /S (00)81692-X)

4. Figure 3
Model of TSC Generation of Germline and Somatic Progenitor Cells
Totipotent stem cells (TSCs) can diverge into GSCs and somatic stem cells (SSCs). A possible source of residual TSC is depicted. Although TSCs have not yet been identified or isolated either in the adult or in the embryo, it shall be important to identify the genes that regulate the outcomes of TSCs to somatic or germline stem cells.
Cell  , DOI: ( /S (00)81692-X)

5. Figure 4
Possible Sources of Tissue-Specific Stem Cells from Another Tissue
In this model, the emergence of, say, HSCs from brain neurospheres could involve either transdifferentiation (brain→blood) or dedifferentiation (brain→TSCs), or by the actions of rare, but residual TSCs.
Cell  , DOI: ( /S (00)81692-X)

6. Figure 5
A Colonial Protochordate that Undergoes Natural Germline and Somatic Stem Cell Transplantation
(A) A Botryllus schlosseri composite colony. Each colony is composed of several systems (flower-shaped), which in themselves are composed of individual “zooids” (petal-shaped). The systems of a colony are joined by a vascular network, which can be seen at the colony margins. In this photograph, a colony bearing orange pigment cells has fused with a colony bearing purple pigment cells, which results in the interconnection of their vasculature networks.
(B) A model of somatic cell and germ cell parasitism following fusion of Fu/HC-compatible colonies. A schematic representation of a transverse section through a blue colony newly fused to an orange colony shows the common vasculature (in white), through which blood cells circulate freely. The progenitor germline cells (circulating cells without an outline) and progenitor somatic cells (cells with black outline) enter all buds (upper left and upper right, respectively, of the blue and orange zooids) of both colonies. The progenitor cells of the two colonies compete for the sites of organogenesis within the buds. In this example, following the next asexual generation, the orange progenitor somatic cells have won over the colonies' somatic progenitors, and the zooid is a genetic clone of the orange colony. Independently, the progenitor stem cells of the blue colony have won over orange for the gonadogenesis sites, and thus, the testes in both colonies are derived from the blue colony. The degenerated zooids from the first generation are shown as fernlike appendages on the proximal flanks of the zooids. The second and subsequent generation colonies retain a mixture of circulating cells from both the orange and the blue colonies.
(C) Schematic representation of fusion and rejection between Botryllus schlosseri colonies. Three Fu/HC-genotyped colonies (A/C, A/B, and C/D) are depicted, with fusion occurring between A/C and A/B and rejection occurring between A/B and C/D. Little or no exchange of circulating cells occurs between the rejecting pairs. Adapted from Magor et al
Cell  , DOI: ( /S (00)81692-X)